In a photodetector, a semiconductor junction, e.g. a p-n or p-i-n junction, is formed at or near the interface of a light absorptive semiconductor body of a first conductivity type and a region of a second conductivity type overlying the body. When a reverse bias voltage is applied to the photodetector, a depletion region, void of mobile charge carriers, is created. Light incident on the photodetector is absorbed creating electron-hole pairs which are swept out of the depletion region thereby generating a detectable electric current. The junction periphery, i.e. the area where a semiconductor junction intersects the surface of the device, is typically characterized by edge breakdown and multiplication of surface leakage currents which substantially increase the dark current, i.e. the reverse bias leakage current which flows with no light incident on the device. This has the adverse effect of providing a less sensitive photodetector.
The incidence of edge breakdown and surface currents has been reduced by fabrication photodetectors such that the second region is a well-like area within the first region. This can be done, by example, by depositing a mask over the top surface of the first region and diffusing a dopant of the opposite conductivity type through an opening therein. This provides a junction which extends to the top surface of the first region under the mask. Preferably, the diffusion mask should also serve as a protective passivating layer since it covers the device surface at the semiconductor junction periphery.
Passivating layers of silicon oxides, e.g. SiO.sub.2, provide a good moisture barrier, but not an ion barrier, for the portion of the semiconductor junction exposed at the surface of the device.
Passivating layers of silicon nitride provide an excellent barrier to ion migration, but are typically characterized by stress-induced cracking and loss of adhesion to the surface.
Silicon oxynitrides, which are used as diffusion barriers on semiconductor devices, typically have unpredictable and varying properties due to the various possible compositions of the oxynitride and, therefore, varying degrees of the above disadvantages.
A more effectively passivated semiconductor device and a method for its manufacture have been sought.